Sloan Digital Sky Survey

December 12, 2007

The Milky Way has a double halo

This illustration shows the discovery that the outer Milky Way is a really a mixture of two distinct components rotating in opposite directions. The inner component of the Galaxy's halo spins clockwise with the Galaxy's rotation at about 50,000 miles per hour. The outer component rotates counterclockwise to the Galaxy at 100,000 miles per hour.

The international discovery team of the Sloan Digital Sky Survey (SDSS-II) used data to demonstrate that the inner part of the halo is more flattened, and dominates the population of stars up to 50,000 light years from the Milky Way's center. The outer halo is more spherical, and dominates the population beyond 65,000 light years from the Galactic center. It may extend out to more than 300,000 light years.

SDSS-II scientists believe that the two components were made of smaller or dwarf galaxies torn apart and accreted into the Milky Way. They also found differences in the chemical compositions of the inner and outer halos.

(Credit - SDSS-II, Masashi Chiba, Tohoku University, Japan)